PURPOSE: To continue the engine operation without any trouble by constantly varying the fuel concentration output at a specific rate until the air-fuel ratio detection signal is inverted with respect to the target air-fuel ratio, when an error of a fuel concentration detecting means is detected, and then fixing the output to the value gained at the inverted time.

CONSTITUTION: According to both the operation state detected by an operation state detecting means and the basic concentration of fuel detected by a fuel concentration detection means, a basic fuel supply quantity setting means sets the basic quantity of fuel supplied During the specified operation, a feedback correction quantity setting means sets the feedback correction quantity as to approach the air fuel ratio detected by an air fuel ratio detecting means to the target air fuel ratio, and finally, a fuel supply quantity setting means sets the quantity of fuel supplied. When an error of the fuel concentration detecting means is detected by an error detecting means, the fuel concentration setting means varies the fuel concentration output at the constant rate until air and fuel mixture changes from the rich to the lean range or vice versa with respect to the target air fuel ratio and finally, fixes the value of the inverted point.